Autonomous Underwater Vehicle

ABSTRACT

Multi-stage buoyancy changing system for an autonomous underwater vehicle comprises: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir and configured to exchange fluid with the internal reservoir via one or more channels; a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a first segment of a dive profile to increase apparent displacement and buoyancy of the vehicle; and a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a second segment of the dive profile to increase an apparent displacement and a buoyancy of the vehicle. The first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile.

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/309,420, filed Mar. 1, 2010, titled Underwater Glider.

The present teachings relate to a multi-stage buoyancy system for autonomous underwater vehicles.

BACKGROUND

Autonomous underwater vehicles that are propelled by changes in buoyancy have become commercial in recent years and demonstrated the ability to operate at sea for long periods. Such vehicles, known in the trade as underwater gliders, are in an early stage of deployment for oceanic research, coastline monitoring, and other applications. While such vehicles have shown viability for many desirable applications/missions, the existing designs are specialized to performing in limited ranges of depths that are optimized to the design of their “buoyancy engine” or buoyancy system. As a result, existing designs are typically optimized for shallow water (e.g., less than 200 meters), deep water (e.g., 200 meters to 1000 meters), or very deep water (e.g., 1000 to 6000 meters). This limits the operation of existing underwater gliders to a specific domain of underwater depth profiles that any specific vehicle can traverse.

Underwater gliders can work, for example, as described in U.S. Pat. No. 3,157,145 to Farris et al., the entire disclosure of which is incorporated herein by reference. A glider can comprise a main body, wings, and an adjustable portion such as an external bladder for changing the apparent displacement of the glider. The external bladder can initially be filled with a fluid such as oil to maximize the buoyancy of the glider when the glider is initially launched in the water. A valve can initially be set in a closed position to prohibit the fluid in the bladder from leaving the bladder. To begin the glider's descent, the valve can be opened, allowing fluid to escape the bladder (for storage in, for example, an internal storage reservoir). As fluid leaves the bladder, the apparent displacement of the glider decreases while the glider's mass stays the same, causing the glider to begin its descent into the water.

As the glider descends, the wings of the glider cause it to move forward. Similarly, the wings cause the glider to move forward as it ascends through the water. To move forward, the glider must typically be ascending or descending in the water. The glider moves forward through its intended path by changing its buoyancy to move up and down through the water, propelling it forward. Because more vertical movement is possible in deeper waters, a greater horizontal distance can be traversed by a glider for a single descent and ascent in deeper waters. Thus, it may be possible to traverse 10 kilometers horizontally in a single dive in deeper water, whereas 10-20 dives can be required to traverse 10 kilometers in shallower water. If the same pump is used in both shallow and deep water, the 10-20 dives can use far more energy (e.g., pumping fluid into the bladder to cause the glider to ascend) than the single dive in deep water. Thus, smaller and more efficient devices such as pistons moving fluid in and out of the external bladder are typically used for gliders used in shallow water.

SUMMARY

The present teachings provide a multi-stage buoyancy changing system for an autonomous underwater vehicle comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a first segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a second segment of the dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle. The first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile.

The present teachings also provide a method for employing a multi-stage buoyancy changing system for an autonomous underwater vehicle having an internal reservoir connected to an external bladder via one or more channels. The method comprises: in a first segment of a dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the first segment of the dive profile; and in a second segment of the dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the second segment of the dive profile. The first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile.

The present teachings further provide a multi-stage buoyancy changing system for an autonomous underwater vehicle comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a pump motor in combination with a continuous variable transmission that can adapt to a torque-speed curve to obtain an optimal pressure/pumping rate needed for a current ambient pressure of the autonomous underwater vehicle, the pump motor and continuous variable transmission being configured to move fluid through a first channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of more than one segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a third channel configured to allow fluid to move from the external reservoir to the internal reservoir, the third channel comprising a solenoid valve that can be selectively opened to allow water to pass from the external bladder to the internal reservoir.

Additional objects and advantages of the present teachings will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present teachings. The objects and advantages of the teachings will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and, together with the description, serve to explain the principles of the teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an autonomous underwater vehicle descending into a deep depth range of a universal glider range.

FIG. 1B illustrates an exemplary dive profile having three distinct depth ranges for which multiple pump stages are utilized during ascent.

FIG. 2 is a chart illustrating pressure versus depth underwater.

FIG. 3 illustrates an exemplary underwater dive profile with time represented on the horizontal axis and depth represented on the vertical axis.

FIG. 4 is a flow chart outlining the basic steps of an exemplary algorithm for implementing a multi-stage system to achieve efficiency at various depth profiles.

FIG. 5 is a schematic diagram illustrating an exemplary embodiment of a hydraulic multi-stage buoyancy system in accordance with the present teachings.

FIG. 6 is a schematic diagram illustrating another exemplary embodiment of a multi-stage buoyancy system in accordance with the present teachings.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the present teachings, examples of which are illustrated in the accompanying drawings.

In many autonomous underwater vehicle implementations, most (e.g., about 75%) of the underwater vehicle's energy can be used to pump fluid (e.g., hydraulic fluid, water, seawater, or other non-compressible fluids or fluids having low compressibility) into an external bladder from an internal storage reservoir to increase the underwater vehicle's apparent displacement and buoyancy to cause the underwater vehicle to ascend to move forward and/or to reach the surface of the water for data receipt and transmission. The amount of pressure required to pump fluid into the external bladder typically varies by depth. For example, in shallow water (e.g., less than about 200 meters) the required pressure can have a magnitude of hundreds of psi, whereas in deep water (e.g., about 200 meters to about 1000 meters) the required pressure can have a magnitude of thousands of psi.

This difference in pressures required to pump fluid into the autonomous underwater vehicle's external bladder has created a design dilemma, because existing pumps that are powerful enough to create enough pressure to pump fluid into the external bladder in deeper water with high ambient pressure are typically inefficient for use in shallower waters with low ambient pressures and in certain dive profiles where the pump must cause the underwater vehicle to ascend more frequently to cover a given horizontal distance. The pressure that must be generated by existing deep water glider pumps makes those pumps less energy efficient. Low pressure pumps are more energy efficient but typically do not provide sufficient pumping force in deeper waters. This design dilemma causes existing autonomous underwater vehicles to be optimized for a limited range of depths.

As stated above, this autonomous underwater vehicle pump design dilemma is imposed by the existence of increasing hydrostatic pressure with increasing depth. For example, a system required to compensate for the hydrostatic pressures that are encountered in water that is typically considered shallow, for example from the surface to a depth of 100 meters, must overcome ambient pressures ranging from 1 atmosphere (14.7 psi) to about 14 atmospheres (200 psi). By comparison, the pressure change that must be overcome by a deep diving vehicle can range from a surface pressure of about 1 atmosphere (14.7 psi) to nearly about 102 atmospheres (1500 psi). FIG. 2 is a chart illustrating pressure versus depth underwater, where the vertical axis represents hydrostatic pressure in pounds per square inch (psi) and the horizontal axis represents depth in meters. A single pumping system that can overcome 1500 psi will not be as energy efficient for pumping lower psi that occurs in shallower depths. In contrast, a buoyancy system designed to handle a smaller range of pressure compensation will use significant less energy to do so. Thus, as stated above, existing autonomous underwater vehicles are offered to be efficient in, generally, one of four ranges: 0 to 30 meters; 10 to 100 meters; 40 to 200 meters; and 200 to 1000 meters. In addition, there is an existing glider capable of diving to 6000 meters, which is not considered to be efficient at depths of less than 200 meters.

A pump capable of producing enough pressure to move fluid into an autonomous underwater vehicle's external bladder in deep water is far less efficient than a pump that is capable of producing enough pressure to move fluid into the external bladder in shallow water. A deep water pump can use, for example, nine times more energy than a shallow water pump. An example of a commercial pump used in deep diving gliders is the Hydro LeDuc model PB32.5 which can pump against 100 atmospheres and requires 14 ft lbs (20 nm) of energy to drive. By comparison a hydraulic pump, such as the MicroPump GB models, requires about 1.25 ft lbs energy to pump against the pressure at 100 meters, about 11 atmospheres. When the Hydro LeDuc is used to pump against the lower pressure (e.g., 11 atmospheres), it uses nearly the same amount of energy as it does when pumping against the 100 atmospheres.

The present teaching provide a universal or increased depth range autonomous underwater vehicle system comprising a multi-stage buoyancy system and a control system that can plan the travel of an autonomous underwater vehicle using a depth profile plan and depth sensors (e.g., one or more pressure sensors and/or one or more acoustic altimeters) or a range and heading determined by, for example, an acoustic modem USBL message to determine which portions of the multi-stage buoyancy system to use to achieve the profile plan while utilizing the least amount of onboard stored energy. By sensing the depth and/or the position of the underwater vehicle in its dive profile (e.g., via sensors including depth/pressure sensors and/or acoustic altimeters), a system in accordance with the present teachings allow a single autonomous underwater vehicle to produce efficient motion covering a broad range of depth, including shallow coastal waters to deep ocean domains.

By incorporating a bypass system, a multi-stage buoyancy system in accordance with certain embodiments of the present teachings achieves high efficiency in buoyancy changes without allowing one stage to compromise or restrict the performance of any other stage. An autonomous underwater vehicle having a multi-stage buoyancy system in accordance with the present teachings, an exemplary embodiment of which is illustrated schematically in FIG. 1A, can traverse a dive profile ranging from shallow coastal water to deep water with a single vehicle, without a significant compromise of energy consumption or reliability that might occur in a design optimized for a narrow range of depths.

FIG. 1B illustrates an exemplary dive profile having three distinct depth ranges for which three pump stages are utilized during ascent. As shown, the dive profile includes a single deep dive having five segments: SURFACE; DIVE; APOGEE; CLIMB, and SURFACE. In a first SURFACE segment, the autonomous underwater vehicle, a position of which is indicated by various dots, starts a surface phase and transmits information including, for example, vehicle health (e.g., all systems self-test and indicate that they are working normally), available onboard energy, a dive log, data from onboard instruments (e.g., chemical compounds in water, optical backscatter, sound detection, salinity, predominant currents, images, other physical properties of the ocean, etc.), and receives information including a dive plan having waypoints (e.g., latitude, longitude, depth, descent rate, and ascent rate), instrument sampling rates, and other parameters associated with controlling instruments (turning them off or when to turn them off). After a dive plan is received, the vehicle can calculate a correct rate and angle of descent based at least in part on the new dive plan. An initial GPS location is taken (GPS1) when the vehicle surfaces or is initially placed in the water and then, after data transmission and receipt of a new dive plan (which presently typically takes about 10-15 minutes (using, e.g., about 10 Watts of energy), another GPS location is taken (GPS2) because the vehicle may have moved during data transmission and receipt. Movement of the vehicle from GPS1 to GPS2 can provide information regarding predominant currents affecting the vehicle. A dive log can comprise data indicating how each dive profile step went. The dive log can also record errors and error mitigation attempts, and can collect instrument data.

Certain embodiments of the autonomous underwater vehicle can remain surfaced without using an electro-motive force, while radio communications and electro-optics perform the above mentioned tasks.

During a second dive plan segment, labeled DIVE, the autonomous underwater vehicle's nose is pointed downward and the vehicle begins its dive phase by beginning a descent into a first depth range (typically without using a pump but rather by letting fluid bleed out of the external bladder to an internal reservoir). After descending through the first depth range, the autonomous vehicle enters a second depth range of the DIVE segment that can be identified, for example, by external pressure sensor readings indicating a depth of the vehicle based on the ambient pressure. In accordance with certain embodiments, during descent, the underwater vehicle can change its angle of descent by changing its pitch angle as needed to follow the requested dive profile.

After descending through the second depth range, the autonomous vehicle enters a third depth range of the DIVE segment that can be identified, for example, by external pressure sensor readings indicating a depth of the vehicle based on the ambient pressure. In certain embodiments of the present teachings in which a bathometric map has been stored, the autonomous underwater vehicle can make sure it has reached a maximum depth set forth in the dive profile and/or avoid collision with the bottom (e.g., using acoustic pings to find the bottom) before beginning an APOGEE dive plan segment. As the underwater vehicle reaches the bottom of its dive, for example in the third depth range, it enters an APOGEE dive plan segment. The APOGEE dive plan segment can include a transition from descent to ascent, wherein the autonomous underwater vehicle levels out (becomes horizontal) and changes its inclination (by, e.g., turning its nose upward for an ascent by shifting a mass within the vehicle) before changing buoyancy by pumping fluid from the internal reservoir to the external bladder to begin its ascent and begin a CLIMB segment of the dive plan.

The CLIMB segment of the illustrated dive plan begins in the third depth range, where a Pump Stage 3 is utilized to pump fluid into the external bladder of the autonomous underwater vehicle, which requires a pumping force sufficient to overcome the ambient external pressure at the underwater vehicle's depth. Pump Stage 3 can comprise one or more pumps optimized for the third depth range (i.e., deep water). As the external bladder fills with fluid, the surface area and thus the buoyancy of the underwater vehicle increase, causing the underwater vehicle to ascend. In accordance with certain embodiments, only a nominal amount of fluid is move to the external bladder—just enough to get a desired rate of rise. As the autonomous underwater vehicle begins to ascend, it may need to change the amount of fluid in the external bladder because, for example, the density (e.g., the salinity) of the water may not be what was originally predicted. Thus, more fluid can be pumped into the external bladder or some fluid can be allowed to bleed from the external bladder to alter the rate of ascent. Adding and removing water from the external bladder can be performed, for example, in a PID loop type of arrangement. In certain embodiments, the system may not allow fluid to be bled from the external bladder to slow the underwater vehicle's ascent, because the vehicle typically eventually hits an area of water in its ascent that slows the vehicle down and makes up for a too-rapid rise. Ocean water density tends to be more uniform near the ocean's bottom. Toward the ocean's surface, the density is more likely to vary, for example due to varying temperature or salinity. Salinity may vary due to, for example, fresh water sources such as rivers, streams, runoff, and rain water.

The underwater vehicle ascends through the third depth range to the second depth range. In the second depth range, the depth and thus the ambient pressure decrease, and a Pump Stage 2 can be used to pump fluid into the external bladder if needed to maintain a desired rate of ascent. Pump Stage 2 can comprise one or more pumps optimized for the second depth range. In accordance with certain embodiments, during ascent, the underwater vehicle can change its angle of ascent by changing its pitch angle as needed to follow the requested dive profile. The underwater vehicle ascends through the second depth range to the first depth range. In the first depth range, the depth and thus the ambient pressure decrease, and a Pump Stage 1 can be used to pump fluid into the external bladder if needed to maintain a desired rate of ascent. Pump stage 1 can comprise one or more pumps optimized for the first depth range (i.e., shallower water). Within the first depth range, for example at about 10 meters or less, another SURFACE segment can begin as illustrated. At surfacing, more fluid can be pumped into the external bladder and the vehicle's mass may be shifted to get the vehicle's tail up to allow an antenna located at the tail to rise for communication.

During the second SURFACE segment, the autonomous underwater vehicle can transmit information including, for example, vehicle health (e.g., all systems self-test and indicate that they are working normally), available onboard energy, a dive log, data from onboard instruments (e.g., chemical compounds in water, optical backscatter, sound detection, salinity, predominant currents, images, other physical properties of the ocean, etc.), and can receive information including a dive plan having waypoints (e.g., latitude, longitude, depth, descent rate, and ascent rate), instrument sampling rates, and other parameters associated with controlling instruments (turning them off or when to turn them off). After a dive plan is received, the vehicle can calculate a correct rate and angle of descent based at least in part on the new dive plan. An initial GPS location is taken (GPS1) when the vehicle surfaces and then, after data transmission and receipt of a new dive plan (which presently typically takes about 10-15 minutes (using, e.g., about 10 Watts of energy), another GPS location is taken (GPS2) because the vehicle may have moved during data transmission and receipt. Movement of the vehicle from GPS1 to GPS2 can provide information regarding predominant currents affecting the vehicle. A transmitted dive log can comprise data indicating how each dive profile step went. The dive log can also record errors and error mitigation attempts, and can collect instrument data. Certain embodiments of the underwater vehicle can remain surfaced without using any energy. The underwater vehicle can also be retrieved after a single dive.

In accordance with certain embodiments, the autonomous underwater vehicle can abort a dive when a problem (e.g., a system error) is detected. When a dive is aborted, the autonomous underwater vehicle can pump as much fluid into the external bladder as possible to reach the surface for retrieval.

When the dive plan requires the vehicle to re-dive without surfacing, the vehicle typically levels out and shifts a mass within the vehicle to point its nose downward before the vehicle allows bleeding from the external bladder to begin to dive again.

To provide an autonomous underwater vehicle that can efficiently traverse a dive profile ranging from shallow coastal water to deep water, the present teachings contemplate a multi-stage buoyancy system comprising, for example, a system employing multiple fluid displacement mechanisms (e.g., a multi-pump system or a system employing a combination of pumps and other fluid displacement systems) to provide efficient movement of fluid at a variety of depths. The fluid displacement systems need not all be the same type of fluid displacement system and can comprise, for example, a piston-driven pump, a systolic pump, a Stirling engine, and/or other suitable devices that can move fluid.

Various embodiments of the present teachings provide a system for changing the apparent displacement or incorporated mass of an autonomous underwater vehicle by displacing fluid within an underwater vehicle comprising two or more stages or subsystems of displacement mechanisms as set forth hereinabove, and a control system that determines an appropriate stage to utilize in the environment that is ambient to the underwater vehicle at any given segment of the underwater vehicle's dive profile.

As stated above, an autonomous underwater vehicle must descend and ascend in the water to move forward and traverse its intended path. FIG. 3 illustrates an exemplary underwater dive profile with time represented on the horizontal axis and depth being represented on the vertical axis. FIG. 3 shows that the ascent phase of the underwater vehicle's dive profile is where the multi-stage control of the present teachings is effective in allowing the underwater vehicle to employ more than one fluid displacement mechanism to move fluid to the external bladder with maximum efficiency while providing the pressure needed to fill the bladder based on the ambient pressure at the underwater vehicle's depth.

At the end of the ascent phase, the autonomous underwater vehicle can reach a surface level (or at least come close enough to the surface) where it can send data (e.g., via satellite transmission) regarding its preceding path and/or begin a new decent and ascent cycle. Upon surfacing, the underwater vehicle can reconcile its location by receiving its current GPS location and inputting that location into its dive profile.

A multi-stage buoyancy system in accordance with the present teachings can be implemented in a number of ways using a variety of approaches that embody the principle of depth-driven and pressure-driven selection of the most efficient stage. For example, by using a pressure sensor that detects the surrounding water pressure at a given depth, the control system or electrical logic of the vehicle can enable the pumping stage that is most efficient for the detected environmental pressure. In principle, a system of many stages can be employed, wherein two is the simplest case for use as an exemplary embodiment herein and may be adequate for many coastal to deep water oceanic missions for autonomous underwater vehicles. The present teachings contemplate, however, more than two stages being used to achieve high efficiency across the entire depth of the ocean from a few meters to 6000 meters or more.

The design principle driving selection of different pumps and pump drive motors for differing depth ranges can be such that the pumping energy for predefined depth ranges and associated pressure is minimized on the basis of balancing the rate of pumping against the torque and hence energy consumption required to resist and overcome the range of pressures within a depth range and move enough fluid to achieve a required buoyancy offset. For example, a depth range of from 0 to 100 meters typically has a corresponding pressure range of from about 1 atmosphere to about 11 atmospheres, and this would dictate that a pump and drive motor capable of most efficiently overcoming the 11 atmospheres maximum value would be selected for this depth range. For a range of 100 meters to 500 meters, having a pressure range of from 12 atmospheres to 50 atmospheres, a stronger pump/motor drive combination is needed, preferably having the best energy efficiency for that range. This design criteria can continue until a maximum depth demanded by the vehicle is serviced by a pump and motor drive stage that meets the maximum pressure demand, while using the minimum energy to achieve buoyancy change by volume of expelled fluid to overcome pressure at any given depth.

Using the above design approach for very large depth ranges can, in certain instances, produce a sub-optimal match of pumping stage to the encountered pressure at some depths, or can produce a design with an excessive number of stages and thus excessive complexity and a significant number of parts lending toward failure modes. Thus, another embodiment of the present teachings contemplates utilizing a pump motor in combination with a continuous variable transmission (CVT) that can adapt to a torque—speed curve resulting in an optimal pressure/pumping rate needed at any given depth of the autonomous underwater vehicle. CVTs can provide an effective continuum of torque-speed ratios over a predetermined range, with slower speeds corresponding with higher torque output. A continuous variable transmission would effectively allow a pump to work across an entire pressure range efficiently by virtue of operating at a faster rate (using a low gear ratio for shallower water, lower ambient pressure, lower torque requirements) or slower rate (using a high gear ratio for deeper water, higher ambient pressure, higher torque requirements) of fluid displacement as needed to minimize the torque and hence the energy required to change buoyancy as needed to allow the underwater vehicle to follow its dive profile.

A CVT-based implementation of the present teachings is practical for increased dive durations associated with increased dive depths. For example, dives to 50 meters will typically take from 15 to 20 minutes, whereas dives to 1000 meters can take up to 5 hours, affording a far longer time frame for the pumping system to move the fluid to achieve ascent velocity when ascending from a 1000 meter depth. In other words, a CVT-based embodiment would pump slowly, using less energy when at greater depths by employing a high gear ratio in the CVT, resulting in low pumping speed but high enough torque to overcome external pressure. Given the longer duration of deeper dives, this can produce an acceptable and optimized result with a single pump design. Where pressures are low in shallow dives, the amount of torque required by the pump to overcome the external water pressure is much lower, but rapid pumping to achieve rapid ascent is typically desirable, so the CVT would then be set to a low gear ratio between the drive motor and the output pump, achieving a higher pumping rate with the lower torque demand. The best effective gear ratio of the CVT for a given ambient pressure can be automatically selected by reading the pressure sensor, then applying an algorithm or other analog control scaling to cause the control arm or other mechanisms that determines the CVT's effective gear ratio to react proportionally or in steps to pressure changes, in a relationship that decreases the effective gear ratio as depth (and hence pressure) increases.

An exemplary embodiment of the present teachings that employs a CVT can utilize a NuVinci® Model N360 continuously variable planetary drive train transmission or another continuously variable or step gearbox mechanism that can vary the pump-to-drive motor effective gear ratio based on a proportional algorithm that is keyed to pressure. As will be understood by those skilled in the art, low gear ratios can be used at shallower depths with lower external pressures, and high gear ratios can be used in deeper waters to produce the extra torque needed to push fluid into the external bladder and against the higher external pressure exerted on the external bladder.

A flow chart outlining the basic steps of an exemplary algorithm for implementing a multi-stage buoyancy system to achieve efficiency at various depth ranges is illustrated in FIG. 4. The flow chart of FIG. 4 illustrates the basic concept of a two-stage system, one stage being for shallow dive segments of the autonomous underwater vehicle's dive profile and the other stage being for deep dive segments of the underwater vehicle's dive profile. The present teachings contemplate using either the profile sequence or the actual pressure to determine which of the fluid displacement mechanisms (e.g., which pump) is used for a given segment of a dive. In certain embodiments, two or more fluid displacement mechanisms (e.g., two pumps or two stages) can be used for a single segment. In certain embodiments, only the ascent phase of a dive, as illustrated in FIG. 3, uses the underwater vehicle's fluid displacement mechanisms to change the underwater vehicle's buoyancy.

As shown in FIG. 4, after a dive sequence begins, the autonomous underwater vehicle performs a next segment of the dive profile, which can be the initial dive profile segment. Each time a new segment of the dive profile begins (e.g., based on a reading of the depth, compass, attitude, or other sensors, singularly or in combination), the algorithm determines whether the profile is an ascent segment (in which one or more fluid displacement mechanisms may need to be employed to displace fluid into an external bladder). If the next segment of the dive profile is not an ascent segment, pressure can be bled from the external bladder to reduce buoyancy, as needed, and the underwater vehicle can begin a descent through the water until a next segment of the dive profile is reached. If the next segment of the dive profile is an ascent segment, the algorithm determines whether the depth of the next segment and the current ambient conditions are less than “Stage 2,” which means that the depth of the next segment and the current ambient conditions are less than a predetermined depth that is optimal for the pump/motor combination currently being utilized, and thus the ambient pressure is below a predetermined value. To determine the ambient pressure, the algorithm can utilize input from a pressure/depth sensor employed on the underwater vehicle. If the depth of the next segment and the current ambient conditions are less than Stage 2, a high pressure fluid displacement mechanism (referred to herein as a “Stage 1 pump”) can be disabled to improve efficiency of the overall system. Thereafter, the system can perform a buoyancy change with just the Stage 2 fluid displacement mechanism and then move on to perform a next segment of the dive profile.

If either the depth of the next segment or the current ambient conditions are greater than or equal to “Stage 2,” which means that the depth of the next segment or the current ambient conditions are greater than or equal to a predetermined depth that is optimal for the next range of pressure and thus the ambient pressure is above a predetermined value, the high pressure fluid displacement mechanism (the Stage 1 pump) can be enabled to provide the pressure needed to move fluid to the external bladder against higher ambient pressures. When the Stage 1 pump is enabled, it can be used alone (by disabling the stage 2 pump as shown), or in conjunction with the Stage 2 pump. The algorithm then performs a next segment of the dive profile. In certain embodiments, two fluid displacement mechanisms can be employed to create a three-stage buoyancy system when each fluid displacement mechanism can be used alone or the two mechanisms can be used together.

In certain embodiments, the control system for the autonomous underwater vehicle uses stored dive profile information, such as the profile illustrated in FIG. 3 or a profile including more than one dive segment such as the segment in FIG. 3, to determine at what time (or distance) an appropriate stage should be used—based on a profiled desired depth. Since this method depends on an accurate assessment of the vertical distance traversed by the underwater vehicle, which can be significantly affected by currents and density structure of the dive environment, certain embodiments of the present teachings can employ a secondary method for selecting the buoyancy stage, such as by reading an external pressure sensor or another method to determine actual depth (e.g., by an acoustic altimeter or by a range and heading determined by an acoustic modem USBL message). The depth and/or depth analog such as pressure can then be used to select the appropriate stage to be used for the desired buoyancy changes in that segment of the dive profile. The terms stages, pumps, and fluid displacement mechanisms are used interchangeably herein.

FIG. 5 is a schematic diagram illustrating an exemplary embodiment of a hydraulic multi-stage buoyancy system in accordance with the present teachings. The exemplary embodiment of FIG. 5 includes, among other elements: a first stage (Stage 1) pump for high pressure depths; a second stage (Stage 2) pump for lower pressure depths; an internal reservoir for fluid (e.g., hydraulic fluid) used to change buoyancy; and a buoyancy chamber or external bladder mounted on an external surface of the underwater vehicle that changes in size and displacement when hydraulic fluid is pumped into it or expressed from it by ambient pressure as the underwater vehicle enters deeper water. The external bladder is preferably at least somewhat elastic.

FIG. 5 illustrates an exemplary embodiment of paths fluid can take between the internal reservoir and the external bladder. In the illustrated embodiment, three paths exist between the accumulator/reservoir and the external buoyancy chamber or bladder, two of which contain a fluid displacement mechanism. One path runs fluid through a low pressure “Second Stage” (Stage 2) pump and through a check valve such as the illustrated bypass (check) valve that prevents movement to fluid in an unwanted direction. Another path runs fluid through a bypass (check) valve and through a high pressure “First Stage” (Stage 1) pump. The parallel channels having bypass check valves can combine to eliminate unproductive loads on a stage that is currently operating, by providing a direct path to the internal reservoir, without the fluid needing to be pushed or pulled through any non-operating elements (e.g., non-operating stages). The third path allows fluid to return from the external bladder to the internal reservoir through a valve such as, for example, a solenoid valve (e.g., a Skinner valve).

To cause the autonomous underwater vehicle to descend, the Skinner valve can be opened between the external bladder and the internal reservoir, allowing fluid to be driven by ambient pressure from the external bladder to the internal reservoir. In the illustrated embodiment of FIG. 5, a return valve such as an electronically actuated solenoid valve (e.g., a Skinner valve) is located between the external bladder and the internal reservoir, although those skilled in the art will appreciate that other suitable types of valves can alternatively or additionally be used. The valve between the external chamber and the internal reservoir should remain selectively closed while the external buoyancy chamber is being filled to cause the underwater vehicle to ascend.

In the illustrated embodiment, a check valve is provided between the line returning fluid from the external bladder to the internal reservoir and the Stage 1 pump. This check valve can prevent fluid returning to the internal reservoir from being diverted to the Stage 1 pump.

While atmospheric pressure can be sufficient to drive fluid from the external bladder to the internal reservoir, certain embodiments of the present teachings also contemplate using one or more of the pumps to drive fluid from the external bladder to the internal reservoir, for example if fluid is not moving therebetween or if fluid is not moving fast enough therebetween to achieve a desired rate of descent.

The illustrated exemplary embodiments of the present teachings eliminate the impact of serial placement of stages by placing the stages in parallel. Serial placement of the stages can impede an optimal performance of stages downstream or upstream in the system. If, for example, a smaller pump was positioned between a larger pump and the reservoir, the smaller pump could restrict the larger pump's access to the reservoir, making it less efficient and/or slower for the larger pump to move fluid from the reservoir to the external bladder. Pumps arranged in series between the reservoir and the bladder, rather than in parallel as illustrated in FIG. 5, would tend to add frictional and orifice (size) restrictions that can impede fluid flow.

Certain embodiments of the present teachings can combine two or more stages to achieve either greater total pressure output to overcome pressures at deeper depths or to increase the rate of change of buoyancy by pumping more fluid into the bladder to increase a rate of ascent. Certain embodiments of a control system for an embodiment utilizing two stages to increase a rate of change in buoyancy can, for example, sense the rate at which buoyancy of the underwater vehicle is being changed, which in some embodiments can be determined by the displacement of an internal plate inside the fluid reservoir, and in other embodiments by, for example, measuring a reservoir pressure. When the rate of buoyancy change reaches or exceeds a level required to achieve the underwater vehicle ascent or descent rate desired in the dive profile, one of the stages can be halted to save energy. The stage to be halted can depend, for example, on the underwater vehicle's depth. For example, if the underwater vehicle is in deeper water requiring use of a stage 1 pump, the stage 2 pump would be halted. Otherwise, if the underwater vehicle is in shallower water requiring use of a stage 2 pump, the stage 1 pump would be halted. If less than all of the stages of the system are being utilized and the rate of buoyancy change falls below a desired level, one or more additional stages can be switched on to provide additional buoyancy fluid flow.

FIG. 6 is a schematic diagram illustrating another exemplary embodiment of a multi-stage buoyancy system in accordance with the present teachings. The exemplary embodiment comprises a main pump and a boost pump for pumping fluid from an internal reservoir to an external bladder. The main pump can comprise, for example, a high pressure pump. The boost pump can comprise, for example, a lower pressure pump. As shown, fluid can travel from the internal reservoir to the external bladder to cause ascent via a first path and/or a second path. The first path includes the boost pump, a filter, and a check valve, the check valve ensuring that fluid flows through this path only in the desired direction. A filter is not essential, but can protect the system from contaminants in the fluid that would decrease the flow or clog the valves. The second path comprises the main pump with a check valve on either side thereof, each check valve ensuring that fluid flows through the second path only in a desired direction.

During a descent, movement from the external bladder to the internal reservoir is called ‘bleeding’ and the ambient underwater pressure is used to push fluid from the external bladder into the underwater vehicle's internal reservoir by pressing on the external bladder. In addition, the autonomous underwater vehicle can have a negative internal pressure that assists the bleeding process and encourages fluid flow back to the internal reservoir when a high pressure (h.p.) return valve between the external bladder and the internal reservoir is opened.

In certain embodiments, the check valve in the second path, located outside the pressure hull, is located within the external bladder and can prevent fluid from flowing back into the pump(s). Because the external bladder is both elastic and exposed to the ambient pressure of the surrounding water, it will experience an internal pressure that tends to push fluid back toward the pump(s). The check valve located outside the hull (e.g., inside the external bladder) serves as a backflow preventer, making the return valve the only outlet from the external bladder. The return valve is selectively openable and only opened when it is desirable to allow fluid to bleed from the external bladder.

In certain embodiments of the present teachings, a flow-through connection can exist through an intake reservoir of the main pump. Pressure from the boost pump can flow to the external bladder until a predetermined ambient pressure of, for example, 200 psi exists. When the predetermined ambient pressure is reached, fluid from the boost pump can be sent (circuitously but effectively) through the main pump's intake reservoir via a flow-through connection and back to the internal reservoir. The flow-through connection thus can function as a safety path.

A path exists for fluid to flow from the external bladder to the internal bladder to cause the underwater vehicle to have a decreased apparent displacement and a decreased buoyancy, and therefore to descend, the path including a return valve such as a electronically actuated solenoid valve (e.g., a Skinner valve) as shown in the embodiment of FIG. 5.

In accordance with certain embodiments, the autonomous underwater vehicle can comprise a variable buoyancy displacement chamber or variable volume enclosure that can be offset from the center of gravity of the underwater vehicle, providing a means to change the displacement volume or the mass of the underwater vehicle relative to its center of gravity, for example to tip the nose of the underwater vehicle up or down. For example, such a mass distribution mechanism can comprises a vehicle battery or another defined mass within the underwater vehicle that can be adjusted within the underwater vehicle to tip the nose of the underwater vehicle up or down, or to roll the underwater vehicle to its left or right. Movement of the mass distribution mechanism can be controlled by the control system, allowing the control system to steer the underwater vehicle as needed to cause the underwater vehicle to descend to desired depths, ascend to the water surface, roll/steer left or right, or keep station as might be determined by the buoyancy of the underwater vehicle relative to the surrounding ambient water and the center of buoyancy of the underwater vehicle.

The present teachings provide a multi-stage buoyancy engine or system in which two or more stages can be combined to increase the rate of buoyancy change as determined by the control system to maintain a desired rate of horizontal and/or vertical velocity for the vehicle in accordance with a predetermined dive profile plan. A bypass system such as the bypass valves disclosed above for the multiple stages of the buoyancy engine or system enables use of one or more stages to obtain optimal energy consumption at a given depth, with no significant impedance or degradation of efficiency imposed by any other stage of the system.

Various embodiments of the present teachings provide an arrangement of multiple stages such that they can be combined to provide a higher rate of buoyancy change or higher torque, whereby a bypass system allows the stages to be provided in parallel. Various embodiments can also comprise a mechanism to change the center of gravity of the autonomous underwater vehicle to cause the underwater vehicle to roll (rotation about the longitudinal axis of the vehicle) and pitch (rotation about the lateral axis of the vehicle), such that the attitude of the vehicle can be changed to provide a desired glide angle relative to forward motion. The external bladder can be used to cause the underwater vehicle to roll and pitch, and can change the center of gravity of the autonomous underwater vehicle.

One or more known energy storage systems onboard the autonomous underwater vehicle can power the fluid displacement mechanisms, the sensors, and the control system. In certain embodiments, the energy storage systems can comprise one or more rechargeable (e.g., lithium) batteries.

As set forth above, various embodiments of the present teachings comprise a control system for an autonomous underwater vehicle, the control system comprising a control computer, sensors to determine depth, heading angles, and rate of descent, and a buoyancy system for changing the apparent displacement or mass of the underwater vehicle using fluid displacement mechanisms to move fluid between an internal reservoir and an external bladder.

Certain embodiments of the present teachings provide an algorithm for determining the appropriate fluid displacement mechanism to use to achieve a desired change in buoyancy to maintain ascent or descent at a specified velocity through a specific range of depths. The fluid displacement mechanisms can comprise a hydraulic system configured with multiple pumping stages or alternate gearing ratios that can efficiently transfer work from one stage to another without significant impairment of a selected stage, and can work in concert or separately to produce changes in buoyancy with respect to the ambient pressure in effect at the time of execution of buoyancy change.

The present teachings provide a configuration of controls, sensors, and fluid displacement mechanisms that can include motors, pistons, or similar mechanisms that enable a change of buoyancy of the autonomous underwater vehicle in accordance with its environment, to minimize its expenditure of stored energy. An advanced method uses a continuously variable transmission to effectively obtain the benefits of a large number of physically separate stages by employing a single stage having continuously changeable torque, flow rate, and pressure outputs.

The present teachings also comprise a control algorithm that can store a desired path of the autonomous underwater vehicle including a depth profile and bathymetric information about the intended path of travel of the vehicle, such that appropriate buoyancy control actions can be programmed to use the most efficient employment of fluid displacement mechanisms to minimize utilization of onboard stored energy.

An autonomous underwater vehicle employing control and buoyancy systems in accordance with the present teachings can travel across long distances (e.g., thousands of kilometers) over durations of many months using buoyancy changes that combine algorithms, controls, and multi-stage buoyancy control to conserve onboard stored energy by utilizing an optimized fluid displacement strategy, selecting the most efficient fluid displacement mechanism(s) to traverse both shallow water and deep water diving profiles.

Other embodiments of the present teachings will be apparent to those skilled in the art from consideration of the specification and practice of the teachings disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims. 

1. A multi-stage buoyancy changing system for an autonomous underwater vehicle, the system comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a first segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a second segment of the dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle, wherein the first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile.
 2. The system of claim 1, further comprising a mass distribution mechanism configured to shift a center of mass of the autonomous underwater vehicle to allow a portion of the autonomous underwater vehicle to surface when a buoyancy of the underwater vehicle is positive.
 3. The system of claim 2, wherein the autonomous underwater vehicle remains surfaced by maintaining positive buoyancy and a shifted center of mass, so that a distal end of the autonomous underwater vehicle is held above a surface of the water while information is transmitted and received.
 4. The system of claim 2, wherein the autonomous underwater vehicle is configured to shift its center of mass to travel horizontally in a neutrally buoyant state.
 5. The system of claim 1, wherein the autonomous underwater vehicle comprises an expandable portion that is capable of withstanding ambient pressures of surrounding water up to a predetermined depth.
 6. The system of claim 5, wherein the autonomous underwater vehicle displaces a volume of water and is configured to expand or contract to increase or decrease, respectfully, the displaced volume of water to control a buoyancy and a center of gravity of the autonomous underwater vehicle.
 7. The system of claim 1, wherein the autonomous underwater vehicle comprises a nose and a tail, the tail comprising a portion configured to rise above a surface of the water that includes one or more of an antenna for radio communication, a GPS locator, and other RF subsystems configured to communicate data that the autonomous underwater vehicle has collected while submerged and obtain a geographical location of the autonomous underwater vehicle.
 8. The system of claim 1, further comprising one or more sensors that collect data while the underwater vehicle is submerged.
 9. The system of claim 1, further comprising electro-optical devices that can be used for above-the-surface reconnaissance when the autonomous underwater vehicle is surfaced.
 10. The system of claim 1, wherein the autonomous underwater vehicle has a front and a rear, and is configured to shift its center of buoyancy and center of mass toward the front or the rear while decreasing and increasing its buoyancy, respectfully.
 11. The system of claim 1, wherein the first device moves fluid from the internal reservoir to the external bladder through a first channel and the second device moves fluid from the internal reservoir to the external bladder through a second channel that is different than the first channel.
 12. The system of claim 11, wherein the first channel and the second channel are arranged in parallel rather than in series.
 13. The system of claim 12, wherein a check valve is located in each of the first channel and the second channel and is configured to prevent fluid from moving from the external reservoir to the internal reservoir through either of the first channel and the second channel.
 14. The system of claim 13, further comprising a third channel configured to allow fluid to move from the external bladder to the internal reservoir.
 15. The system of claim 14, wherein the third channel comprises a solenoid valve that can be selectively opened to allow water to pass from the external bladder to the internal reservoir.
 16. A method for employing a multi-stage buoyancy changing system for an autonomous underwater vehicle having an internal reservoir connected to an external bladder via one or more channels, the method comprising: in a first segment of a dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the first segment of the dive profile; and in a second segment of the dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the second segment of the dive profile, wherein the first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile.
 17. The method of claim 16, further comprising shifting a center of mass of the autonomous underwater vehicle so that a nose portion of the underwater vehicle is raised before increasing the apparent displacement and a buoyancy of the autonomous underwater vehicle.
 18. The system of claim 16, wherein the first device moves fluid from the internal reservoir to the external bladder through a first channel and the second device moves fluid from the internal reservoir to the external bladder through a second channel that is different than the first channel and is arranged in parallel with the first channel.
 19. The system of claim 18, wherein a check valve is located in each of the first channel and the second channel and is configured to ensure that fluid does not move from the external reservoir to the internal reservoir through the first channel and the second channel.
 20. A multi-stage buoyancy changing system for an autonomous underwater vehicle, the system comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a pump motor in combination with a continuous variable transmission that can adapt to a torque-speed curve to obtain an optimal pressure/pumping rate needed for a current ambient pressure of the autonomous underwater vehicle, the pump motor and continuous variable transmission being configured to move fluid through a first channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of more than one segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a third channel configured to allow fluid to move from the external reservoir to the internal reservoir, the third channel comprising a solenoid valve that can be selectively opened to allow water to pass from the external bladder to the internal reservoir. 